The present invention relates to thermally insulated heating elements. The invention is also directed to improvements in electric toasters with regard to thermal insulation of the wall of the exterior housing, or case.
The usual electric toaster includes an outer case and a toasting chamber that contains a movable bread rack that supports bread slices during toasting. Careful attention must be paid to safety conditions, particularly concerning the risk to users of sustaining burns as a result of possibly touching the outer case. There is also a trend toward toasters having ever smaller dimensions while being capable of operating at high power levels.
While the internal dimensions of the toasting chamber are fixed and determined as a function of the size of the different slices of bread to be toasted, the dimensions of the related heating elements can be varied. In general terms, a toaster has heating means that can function either by radiation or by conduction.
In the case of radiation, the bread to be toasted is brought opposite radiating heating elements and thus receives the quantity of energy necessary for toasting. In the case of heating by conduction, the bread to be toasted is brought into contact with the elements for heating by conduction during a time necessary to obtain satisfactory toasting. In both cases, two main heating elements are presently used, in the form of resistive heating elements with or without a quartz tube and in the form of heating elements composed of resistance strips, or ribbons.
French patent document FR 2175783 describes a toaster having heating elements constituted by sheets of mica on which resistive ribbons are wound, located at both sides of the toasting chamber. Between the mica sheets and the outer case, there are disposed reflectors and/or insulating elements that constitute a protective screen with the respect to the outer case.
This compact form of construction for the heating elements does not, however, guarantee a good thermal insulation because the rear faces of the mica sheets radiate to a substantial extent in the direction of the reflector and the outer case. The reflector plays its role by reflecting a part of the radiation, but it also is heated and transmits, in turn, a part of the radiation toward the outer case. Added to these thermal phenomena are convection flows existing between the mica sheets and the reflector, as well as between the reflector and the outer case. This can lead to a substantial heating of the outer case unless a substantial spacing is provided between the case and the reflectors. As a result, a toaster of this type is not particularly compact.
A second known toaster, marketed by the company Taurus under the product name My Toast®, has two heating elements constituted by resistive ribbons placed between two mica sheets, situated at both sides of the toasting chamber. This toaster has, in succession and when viewed in cross-section, from the center of the toasting chamber: a grid, a first thin mica sheet, a second, thicker, mica sheet wound with the resistive ribbon, a sheet metal reflector and an outer case made of plastic. A metal roof is provided so that the outer case need not be curved to extend over the top of the toaster, which is normally the hottest zone.
However, this form of construction does not permit insulation of the toasting chamber or reduction of the temperature of the outer walls.
The patent document FR 1596661 describes a toaster having a case composed of two distinct parts, composed of an outer part and an inner part that are fitted together in a manner to provide therebetween a space in which air circulates by convection to cool the wall of the outer part.
However, the additional volume of the outer part and the air circulation space serve to increase the overall size of the toaster.